Lhuyd still provides the only guide to native Cornish pronunciation that even approaches clarity and, again, I recommend Richard Gendall's "Guide to the Pronunciation of Cornish" (Teere ha Tavas 1992), which is an analysis of Lhuyd. It might not be perfect, but it's all we have from when the language was still spoken in the community. Why some won't entertain this utterly defeats me.

Lhuyd has the following spellings for the verbal noun: <klyụez, kloụaz, kleụez, glẏụez>
The monosyllabic forms (3sg.pr.fut./2sg.imper.) he spells: <gleụ, glow>

I interpret his forms as clewes ~ clowes [ˈkləwɐz] and clew ~ clow [klɛˑʊ] or [klɔˑʊ].

And _klyw_ or _klew_ may well be one such word. It's purely chance that the correction, if judged necessary, happens in this case to take us back to Nance's form.

Yes, and no! UC is Nance's standardisation of the MC textual spellings, mainly the Ordinalia, but also PA, BM and a little CW. That Nance's for of clowes was "correct" demonstrates how important the textual spellings are as opposed to relying too much on forms reconstructed from Welsh and Breton in preference to the textual spellings. That doesn't mean that I don't believe that the textual spellings shouldn't be critically examined, evaluated and interpreted, but if a reconstructed form like KK klywes is so markedly different from the (well attested) MSS forms then this should tip off the phonologist doing the reconstruction. The texts remain our most important corrective, not only, perhaps imperfectly understood phonological equivalents between Cornish, Welsh and Breton.

This doesn't in any way validate Nance's "inspired guesswork" approach to orthography.

"Inspired guesswork" isn't perhaps the ideal term for this. I prefer "standardisation". He wasn't really guessing anything, he was spelling as the texts spelt.

Although as I've tried to explain above, Nance can hardly be blamed. He is of his time, working with the outlook and available techniques of his time.

Not quite, as his day and age was the boom-time of Indo-European historical linguistics and reconstruction techniques, though they have been refined since then, were essentially the same s they are today.

But we are of a later time, and with better techniques and building on the accumulated work of the past we can do better. And I would say we need to do better, because spoken Cornish is now much more to the fore than it was 80-odd years ago. And with the increasing ease with which recordings and videos can be made and distributed, that trend is bound to increase.